1. Field of the Invention
This invention relates to a reed switch unit used for detecting a liquid level of a reservoir tank of a master cylinder.
2. Description of the Related Art
A brake device, used in a vehicle, has such a construction that a brake liquid is circulated in a hydraulic circuit provided between a master cylinder, connected to a brake pedal or the like, and a wheel cylinder connected to a wheel brake. A reservoir tank, which holds the brake liquid so as to supply it, is mounted on the master cylinder.
In order to always ensure a proper operation of such a brake device, it is necessary that the brake liquid should always be stored in an amount, not smaller than a predetermined amount, in the reservoir tank. Therefore, a liquid level detection sensor for detecting the position of the liquid level of the brake liquid stored in the reservoir tank, is provided at the reservoir tank so that an alarm can be given to the driver or the like when the brake liquid in the reservoir tank decreases to an amount smaller than the predetermined amount.
The liquid level detection sensor comprises a float installed with a magnet, which can move upward and downward in accordance with the upward and downward movement of the liquid level, and a reed switch unit including a reed switch for detecting the upward and downward movement of the float.
The reed switch is an electrical part in which a contact is closed when the magnet in the float approaches this reed switch.
A conventional reed switch unit is shown in FIGS. 10 and 11.
The reed switch unit 10 comprises a reed switch 11, and a unit main body 12 of a synthetic resin on which the reed switch 11 is mounted.
In the reed switch 11, a contact portion 14 is formed such as to be sealed in a glass tube 16, and a contact of this contact portion 14 is closed when a magnet approaches the reed switch. Inert gas (e.g. nitrogen gas) is sealed in the glass tube 16 to prevent oxidation and dew condensation from developing on the contact portion 14.
Reed portions 18 are provided respectively at opposite ends of the glass tube 16 so as to extend in an axial direction. The reed portions 18 are connected to the contact portion 14.
A right portion (in the drawings) of the unit main body 12, made of a synthetic resin, is formed into a connector portion 12a connectable to other electrical circuit, whereas a left portion thereof is formed into a mounting portion 12b on which the reed switch 11 is mounted.
Two metal terminals 20 and 20 are provided at the mounting portion 12b, and the two reed portions 18 and 18 of the reed switch 11 are connected to these metal terminals 20 and 20, respectively.
Each of the terminals 20 comprises a plate-like body which extends upwardly from the mounting portion 12b, and has a slit 22 in which the reed portion 18 is fitted. The terminals 20 are connected respectively to connection terminals 23 indicated in broken lines at the connector portion 12a.
The two reed portions 18 and 18 of the reed switch 11 are fitted respectively in the slits 22 and 22, formed respectively in the terminals 20 and 20, and then are soldered to the terminals 20 and 20, respectively, thus fixing the reed switch 11 to the unit main body 12. Since the reed portions 18 are fixed respectively to the terminals 20 by soldering, the physical fixing and electrical connection of the reed portions 18 are both effected positively.
A rib 24 is formed integrally with the unit main body 12, and projects upwardly from that portion of the mounting portion 12b, on which the glass tube 16 is to be disposed, in such a manner as to surround side surfaces of the glass tube 16. The rib 24 serves to position the glass tube 16 when fixing the reed switch 11 to the terminals 20.
With the rib 24 thus provided, when mounting the reed switch 11, the glass tube 16 is first received within the rib 24, and then the reed portions 18 are fitted respectively in the slits 22 of the terminals 20, and then are secured thereto by soldering, so that the reed switch 11 is positively fixed at the predetermined mounting position (see, for example, Japanese Patent Unexamined Publication No. Hei. 7-220817).
In the fixing method as described the above, the glass tube 16 may contact the rib 24 surrounding it. Therefore, when the reed switch unit 10 is mounted on a vehicle, there arises a problem that stresses due to vibrations and thermal expansion and thermal contraction develop in the glass tube 16, so that the glass tube 16 can be damaged or deformed.
Besides, the glass tube 16 of the reed switch 11 to be mounted on the unit main body 12 has a dimensional irregularity, and it is inaccurate to perform the positioning of the contact portion 14 by using the glass tube 16 as a reference, to thereby invite a problem that variations occurs in the detection of the liquid level.
Therefore, this invention has been made in order to solve the above problems, and an object of the invention is to provide a reed switch unit in which damage of a glass tube due to stresses is prevented, and also when fixing a reed switch to a unit main body, the positioning of the reed switch is not performed using the glass tube as a reference.
The above object has been achieved by the following constructions of the invention.
According to the invention, there is provided a reed switch unit for detecting a liquid level of a reservoir tank of a master cylinder, including: a reed switch including a contact portion sealed in a glass tube, and reed portions extending respectively from opposite ends of the glass tube in a direction of an axis thereof; and a unit main body including terminals which support the reed portions, respectively, in electrically-connected relation thereto to fix the reed switch, wherein the reed switch is fixed to the unit main body in such a manner that the glass tube is disposed in non-contact relation to the unit main body.
With this construction, the glass tube does not contact any portion of the unit main body, and thermal stresses, which are produced by heat when mounting the reed switch on the unit main body, and stresses due to vibrations, developing after mounting the reed switch unit on a vehicle, will not be applied to the glass tube, and therefore the glass tube can be prevented from damage.
According to another aspect of the invention, there is provided a reed switch unit for detecting a liquid level of a reservoir tank of a master cylinder, comprising: a reed switch including a contact portion sealed in a glass tube, and reed portions extending respectively from opposite ends of the glass tube in a direction of an axis thereof; and a unit main body including terminals which support the reed portions, respectively, in electrically-connected relation thereto to fix the reed switch, wherein the position of mounting of the reed switch on the unit main body is set by determining the positions of the reed portions.
With this construction, the positioning of the reed switch in its mounting position is effected not by using the glass tube, but by using the reed portion. Therefore, damage, deformation and so on of the glass tube can be prevented, and besides the inaccurate positioning by the use of the glass tube, subjected to dimensional variations, is thus eliminated, and therefore variations in the detection of the liquid level in the reservoir tank can be suppressed.
In the reed switch unit, a stopper wall is formed on the unit main body, and the position of mounting of the reed switch in the axial direction is determined by abutting one end of the reed portion against the stopper wall, and the terminal has a fitting portion which receives the reed portion in such a manner as to determine the position of mounting of the reed switch in a radial direction.
With this construction, when mounting the reed switch on the unit main body, the positioning of the reed switch in the axial direction can be effected by abutting the reed portion against the stopper wall, and the positioning in the radial direction can be effected by fitting the reed portion in the terminal.
The reed portion of the reed switch can be bent at that portion thereof to be fixed to the terminal, and the bent portion is abutted against the terminal, thereby determining the position of mounting of the reed switch. With this construction, when mounting the reed switch on the unit main body, the positioning of the reed switch in the axial and radial directions can be effected by engaging the bent portion of the reed portion with the terminal.
Preferably, the reed portions are fixedly secured respectively to the terminals by resistance welding. With this construction, the electrical connection and physical fixing of the reed switch unit can be positively effected while assuring that the glass tube will not contact the unit main body.